1. Field of the Invention
The present invention relates to a rotary discharge type centrifugal separator wherein feed slurry containing solids is subjected to a centrifugal force to continuously discharge clarified liquid and sludge separately.
2. Discussion of Background
A conventional scroll discharge type centrifugal separator provided with a helical screw conveyor having a frusto-conical end type scraping section or beach zone for scraping solids has been widely used as a centrifugal separator for separating solids and liquid from feed slurry containing a large amount of solid components.
In such a scroll discharge type centrifugal separator, clarified liquid is moved as an inside layer and sludge is moved as an outside layer in a revolving bowl due to the densities, and the clarified liquid is discharged outside from a large diameter end of the bowl, and the sludge is discharged outside from a small diameter end in a frusto-conical end portion of the revolving bowl.
However, when the conventional centrifugal separator is used for separating solids such as paste-like material, e.g. the separation of solid and liquid from reaction liquid of a biochemical reactor, the separation of suspended solids in fruit juice, the separation of colloidal particles in a fine chemical field and so on, the following disadvantages occur. Namely, in the scraping section or the beach zone of the frusto-conical portion of the scroll type centrifugal separator, sludge is moved in the direction of the small diameter end against a slip force vector along the tapered surface of the beach zone in a vector of centrifugal force. Accordingly, when the gradient of the tapered surface of the beach zone is made large in order to reduce the length of the revolving bowl, the slip force also becomes large in proportion to the increased gradient. When the gradient of the inclined portion is made small to reduce the slip force, the length of the revolving bowl becomes long, whereby it is difficult to rotate it at a high speed, and cost for manufacturing the centrifugal separator is increased.
Further, when solid components in the sludge are of a nature which makes it difficult to cause deformation by a shearing force, the scraping function of the screw conveyor is effectively performed and the solid components can be scraped effectively in the frusto-conical beach zone. However, if the solid component is of a soft and paste-like material such as a product from the biochemical reactor, they leak from the air gap between the revolving bowl and the screw conveyor, or they slip along the helical blade formed on the screw conveyor, whereby an effective scraping function can not be obtained.
In order to eliminate the above-mentioned disadvantage, the inventors of this application have proposed a rotary discharge type centrifugal separator as shown in FIGS. 7 and 8 (Japanese Unexamined Patent Publication No. 59065/1992). Namely, the tapered portion of the revolving bowl is eliminated so that the general shape of the revolving bowl is cylindrical. The conical helical blade of the screw conveyor is also eliminated so that the general shape is in a cylindrical helical blade. The scraping portion is formed in a portion at the sludge discharge side of the revolving bowl., Scrapers extended in the radial direction are mounted on an end portion of the screw conveyor shaft so that the sludge is scraped in the circumferential direction at the inner surface of the revolving bowl.
The proposed centrifugal separator will be described in more detail with reference to FIGS. 7 and 8. In FIG. 7, reference numeral 1 designates a revolving cylindrical bowl supported horizontally. The front and rear ends of the bowl are closed by a front end plate 2 and a rear end plate 3 each provided with a hollowed journal projecting from its center. The revolving cylindrical bowl 1 is defined by a separating zone 1a which extends from the front end plate 2 to a portion near the rear end plate 3, and a scraping section 1b as the remaining portion which has a slightly larger diameter.
As shown in FIG. 8, a guide member 5 which defines a so-called cocoon-shaped guide surface 4, i.e. the diametrically opposing long portions of an elongated circle are recessed, is fitted to the inner surface of the revolving cylindrical bowl at the scraping section 1b. Sludge discharge ports 6 are formed at the recessed portions to penetrate the cylindrical bowl in the radius direction of it. Numeral 7 designates a screw conveyor shaft which is coaxially journaled to the revolving cylindrical bowl 1 by means of front and rear end journals and bearings received in the front and rear end plates 2, 3. A helical blade 9 is formed on the outer surface of a hollowed shaft portion 8 of the screw conveyor shaft 7 so as to extend over the separating zone 1a. The outer edge of the helical blade 9 faces, with a small air gap, the inner surface of the separating zone 1a of the revolving cylindrical bowl 1. In the scraping section 1b, a pair of rectangular scrapers 10 are supported symmetrically in the diametrical direction by respective piston rods which are slidably and outwardly extended in the radial direction, and the outer edge of each of the rectangular scrapers 10 is brought into slide-contact with the cocoon-shaped guide surface 4 of the guide member 5.
Numeral 12 designates a disk-type baffle plate fixed to the screw conveyor shaft 7 at the boundary between the separating zone 1a and the scraping section 1b so that the outer periphery of the disk-type baffle plate faces the inner surface of the revolving cylindrical bowl with a small air gap. Numeral 15 designates a casing surrounding the revolving cylindrical bowl 1, numeral 16 designates a pair of main bearings for supporting the hollowed journal portions of the front and rear end plates 2, 3 of the revolving cylindrical bowl 1 to the front and rear ends of the casing 15, numeral 17 designates a feed pipe inserted in the hollowed shaft portion 8 of the screw conveyor shaft 7, numeral 18 designates a clarified liquid discharge port formed at the front end of the casing 15 to project downwardly, numeral 19 designates a condensed slurry discharge port formed at the rear end of the casing 15 to project downwardly, numeral 20 designates a motor and numeral 21 designates a reduction gear.
In the above-mentioned centrifugal separator, feed slurry F supplied to the revolving cylindrical bowl 1 through the feed pipe 17 is subjected to centrifugal separation in a usual manner in the separating zone 1a, and a liquid component L is discharged as clarified liquid through the clarified liquid discharge port 18. As the liquid component is separated, a solid component S is condensed. The solid component S is moved to the scraping section 1b while it is gradually condensed, by means of the helical blade 9 of the screw conveyor shaft 7. The condensed solid component S is forced to enter into the scraping section 1b through the air gap between the disk-type baffle plate 12 and the inner surface of the revolving cylindrical bowl 1. In the scraping section 1b, the condensed solid component S is scraped to get over a beach zone Z by means of the scrapers 10 which are in slide-contact with the cocoon-shaped guide surface 4, whereby the solid component S is discharged from the sludge discharge port of the revolving bowl.
The conventional centrifugal separator can discharge stably such sludge that solids contained therein is of paste-like soft material. However, it has been found that the scrapers supported by the arms having a piston rod structure has a disadvantage in that it is difficult to obtain a sludge scraping function smoothly for a long period of time because the piston rod structure has a sliding portion, and the scrapers are in slide-contact with the guide surface, whereby there is a large bending moment to the piston rod and there is abrasion and friction loss in the system.